This work is directed toward elucidation of the relations between the molecular structure and the allosteric function of hemoglobin. A related aspect of the work concerns molecular aspects of the gelation of deoxyhemoglobin S. A method of assay of gelation of deoxyhemoglobin S by analytical ultra-centrifugation has been developed, dependent on the observation of a phase change at a critical hemoglobin concentration, the minimal gelling concentration. The characteristics of the equilibrium distribution of hemoglobin at concentrations below the phase change offer information on intermolecular interactions. Data show that lowering pH toward 6.5 and the presence of 2,3 diphosphoglycerate favor gelation while high ionic strengths inhibit it. This method is applicable also to studying properties of allosteric conformational isomerization in hemoglobin, which isomerization is critical for the oxygen binding properties of hemoglobin. Other work in progress includes study of the effects of a variety of cofactors which influence oxygen affinity. The specific functional groups of these cofactors interact with specific groups on hemoglobin and thus sterochemical bases of oxygen function may be elucidated.